Si semiconductor and gallium arsenide semiconductor are currently in use with the problem that micronizing and densifying these devices gives rise to a rise in electric field strength in their interior and heat buildup is encountered in their use. Henceforth these semiconductors will have to adapt themselves to meet severe environments.
In contrast, diamond is a wide band gap semiconductor, is the highest in the mobility of electrons and positive holes, is extremely high in breakdown electric field and yet is the least to produce electron and positive hole pairs at an elevated temperature or as influenced by a radiation. Diamond semiconductors, thus with the ability by nature to meet severe environments, are usable in high power, high frequency operating and high temperature operating devices. To realize such a diamond semiconductor device, thin films of diamond crystal of good quality are required.
So far, it has been possible to fabricate low resistance p-type semiconductor diamond by boron dopin with ease. As for low resistance n-type semiconductor diamond, however, it has practically been difficult to obtain a thin film of semiconductor diamond crystal of satisfactory quality, despite the studies that have been pursued to investigate into a large number of manufacturing processes including doping into CVD diamond.
For example, it has been reported that doping diamond with nitrogen reduces its activation energy and makes it an insulator at a room temperature (Mat. Res. Soc. Symp. Proc. 162, 3-14, 1990). Further, a thin film of n-type diamond crystal doped with phosphorus has been reported, which is too high in electrical resistivity, however, to the extent that it is unsuitable for practical use (Mat. Res. Soc. Symp. Proc. 162, 23-34, 1990).
Also, an experiment to obtain an n-type diamond thin film from methane and hydrogen sulfide by a microwave plasma CVD technique has been reported (JP P 63-302516 A). However, as is apparent from Tables 1 and 2 of this patent literature, the n-type semiconductor diamond thin film made using the microwave plasma CVD process has an electron mobility that is abnormally high compared with that of an n-type semiconductor diamond crystal made by an extra-high pressure process shown in Table 2 in spite of the fact that both have an equivalent sulfur concentration. That is to say, it is shown that this n-type semiconductor diamond thin film is deficient and not applicable in a semiconductor electronic device.
Further, in the fabrication of phosphorus doped diamond by the microwave plasma CVD technique, there have been known a phosphorus doping process that introduces phosphine into a reaction gas of hydrogen and hydrocarbon and decomposes phosphine in a microwave plasma, and another phosphorus doping process that decomposes phosphine at a high temperature or by irradiation with an ultraviolet ray.
However, the preceding microwave plasma CVD process because it dopes diamond with phosphorus in a state that it is combined with hydrogen will not make phosphorus a source of supply of electrons and, if phosphorus is doped, only yields n-type semiconductor diamond that is low in carrier mobility and deep in level. For these reasons, no n-type semiconductor of this type that is of the quality which makes it applicable to a semiconductor electronic device has been obtained.
Therefore, as far as the CVD technique is concerned, while thus far there have already been reports about methods tried to make a thin film of n-type semiconductor diamond as mentioned above, none has as yet been obtained thereby that is of a quality level enough to justify its use in a semiconductor electronic device.
Besides, a method has also been known that implants diamond with phosphorus ions accelerated. According to this method, phosphorus that is greater in mass than carbon is implanted, which creates a defect in diamond. Further, because phosphorus without combining with carbon is interstitially included in the diamond lattice, it is more than hard to create such a combination therein. Thus, any such n-type semiconductor of acceptable quality has not as yet been obtained by this method either.
Further, there has also been known the chemical transport reaction process that dopes diamond with phosphorus, which places graphite and red phosphorous in the reaction system and evaporates them in the system to synthesize diamond while doping it with phosphorus.
However, a difference in the rate of reaction and the rate of evaporation between graphite and red phosphorous makes it hard for the process to control the phosphorus concentration. Indeed, no such n-type semiconductor of satisfactory quality sought has been obtained by this process either.
Also, there has recently been proposed an n-type diamond semiconductor having atoms of a valence number of 5 or more added thereto as donor atoms. See JP P 10-194889 A. However, a phosphorus added n-type semiconductor diamond applicable to a semiconductor electronic device has remained unrealized and a method of its making has become a problem awaiting solution.
This invention is aimed to solve such problems encountered in the prior art and has for its first object to provide an n-type semiconductor diamond having a perfect crystallinity that is applicable to a semiconductor electronic device. The present invention also has for a second object to provide a method that enables making an n-type semiconductor diamond which having a perfect crystallinity is applicable to a semiconductor device.